Broadly, the invention relates to control of a gas turbine. More specifically, the practice of the invention involves the use of a sight tube assembly, in combination with a sensing instrument, to continuously monitor the rotating blades in a gas turbine.
Utility companies and other industries use large, stationary gas turbines to drive generators, pumps, or other types of machines. In the turbine section of the engine there are several rows of cup-shaped blades, known as turbine blades, which are mounted on a rotor shaft. A gas heated to extremely high temperatures (often above 1800.degree. F.) is directed against the turbine blades, causing them to rotate and thus drive the rotor shaft. Before each row of rotating turbine blades is a row of stationary guide vanes, which are a part of the turbine section structure. As the hot gas moves through the turbine section, each row of guide vanes directs the flow of the gas so that it impinges on the rotating blades in the next row at the proper angle.
Power output and fuel (thermal) efficiency of the gas turbine increase as the firing temperature increases. The service life of the turbine components (turbine blades and guide vanes) decreases with increasing temperature. There is thus an optimum firing temperature for which power production is maximized, fuel consumption (per unit of power) is minimized and the turbine components retain their integrity for their full design life.
The first row of guide vanes and the first row of turbine blades operate at higher temperatures than do the successive rows of guide vanes and turbine blades. (The hot gas cools as it expands from the high pressure in the turbine engine to the near-atmospheric pressure in the turbine exhaust duct.) At present there is no reliable method to directly monitor the temperatures of the first row of guide vanes and the first row of turbine blades in large industrial gas turbines. Thermocouples are short-lived at such temperatures. Also, they are not easily adapted to measure the temperature of the rotating turbine blades, which historically have been the components whose failure has caused the gas turbine engines to be seriously damaged when overfired. Thus, it is most desirable to directly monitor the metal temperature of the first row of rotating turbine blades.
At present, the average firing temperature is usually calculated in a control processor, which receives as input data the average turbine exhaust temperature and the compressor-discharge pressure of the gas turbine. The control processor modulates the fuel supplied to the combustor (or combustors) in the engine, thus controlling the gas temperature at the inlet of the turbine section of the engine.
Optical pyrometers are presently used in many military aircraft gas turbine engines. These pyrometers generally employ a fiber-optic conduit, which terminates in the engine, a line of sight to the blades being provided through components of the turbine section of the engine. Components of the optical pyrometer system internal to the engine are serviced when the engine is not in operation.
One of the better known prior optical pyrometer systems which may be used for measuring blade temperatures and controlling operation of gas turbines has been developed by Land Turbine Sensors, Inc. At present the Land pyrometer systems are used with fiber optics primarily for in-flight temperature monitoring of engines on jet aircraft, and for obtaining blade temperature profiles in jet aircraft and other turbine engines mounted on test stands. There are several other pyrometer systems available which can be used for measuring turbine blade temperatures, but Applicant is not aware of any system that is readily adaptable to industrial-size gas turbines.
Some of the drawbacks of the known optical pyrometer temperature measuring systems will now be discussed. Using the Land pyrometer system as a typical example, the transducer of this instrument comprises a fiber optic head, a flexible light guide and a detectoramplifier module. The sight tube component of this system is mounted on the turbine engine, such that the lower end passes through the turbine section of the engine housing and fastens into an opening between two vanes in the first row of the stationary guide vane section. The fiber optic head is connected into the upper end of the sight tube, and the probe for the optic head extends downwardly into the sight tube. At the lower end of the probe is a viewing lens, which is positioned a short distance behind the sight tube opening through the guide vane section. This allows the pyrometer to "view" the first row of the rotating turbine blades.
The sight tube includes an air purge inlet near its upper end. Air for cooling the probe and purging the lens is directed through the inlet and flows downwardly through a small annular space between the probe and the inside of the sight tube. The flexible fiber optic light guide is connected at one end into the optic head and at its opposite end into the detectoramplifier module, the module being enclosed in a housing and mounted in a remote location.
A major disadvantage of the Land system is that some of the transducer components must operate within the severe environment of the turbine engine, where they are subjected to high temperatures and pressures, vibration, and contaminants. Because these components are positioned within the engine environment, the engine must be shut down to repair or replace such parts. The penetration of the lower end of the sight tube through the first row of the stationary guide vane section is particularly undesirable in large industrial turbines, because of the difference in construction from aircraft gas turbine engines.
Another drawback of the Land system is that the line of sight from the optic head to the first row of turbine blades is restricted to a given size and position. In addition, the snug fit of the probe inside the sight tube does not allow for adjustment of the line of sight to move the target spot to different locations on the rotating blade surfaces. This limitation is undesirable, since the "fixed" target may not be the hottest part of the turbine blade. For example, scale buildup on the turbine blades can have an adverse effect on the temperature readings Therefore, if the target spot falls on a part of the blade that is scaled over the temperature readings will probably be inaccurate.
The optical pyrometer and sight tube assembly of this invention has distinct advantages over the Land pyrometer system and the other systems described above for measuring blade temperatures in turbine engines. For example, the pyrometer and detector components of this invention are fastened to the sight tube at a position that is completely outside the high temperature and high pressure environments of the turbine engine. The sight tube is positioned in the turbine such that the pyrometer unit has a direct line of sight to the first row of the turbine blades. At the same time, the line of sight can be moved within the sight tube, so that the pyrometer can scan the blade surfaces to find the hottest spot, or the coolest spot. This invention also includes a means for isolating the pyrometer unit from the engine environment, to perform service work, or to remove the pyrometer, without shutting the turbine down. Another advantage of this invention is that the sight tube structure provides an excellent view of the rotating turbine blades without penetrating the stationary guide vane section or other parts of the turbine section structure.